The theme of investigations conducted within this award has been that the pathways which mediate and regulate the apoptotic naturally occurring cell death event during the development of dopamine neurons of the substantia nigra (SN) have potential relevance to the maintenance of viability of these neurons in adulthood, and thus to the pathogenesis of Parkinson's disease. We propose that understanding these developmentally important pathways will provide a scientific basis for the development of neuroprotective approaches to this disease. In this renewal application, we have two principal goals related to this theme. First, in Specific Aims I and II, we seek to determine the physiologic role of the Akt pathway in the development of SN dopamine neurons, and in the maintenance of their viability and functional integrity in adulthood in vivo. We further seek to explore the potential neuroprotective and restorative roles of Akt signaling in models of parkinsonism. This goal acknowledges the extensive evidence obtained in the in vitro context that Akt is a critical survival signaling mediator, and recognizes that there is minimal information available about its role in vivo. In addition, this goal is based on preliminary data which indicates a striking trophic effect of a constitutively active form of Akt on adult SN dopamine neurons, and an ability to substantially protect these neurons in a neurotoxin model. Our second goal is to address in Specific Aims III and IV two important fundamental issues in the neurobiology of glial cell line-derived neurotrophic factor (GDNF) for dopamine neurons. We have shown in the current funding period that GDNF may serve as a physiologic target-derived neurotrophic factor for dopamine neurons during the first phase of natural cell death, based on acute experiments with neutralizing antibodies. However, an important fundamental issue that must be addressed is whether selective loss of the GDNF receptor, GFRal, in SN dopamine neurons results in a lasting decrement in their number in the chronic setting. To address this issue, we have in the current funding period created mice with a floxed GFRal allele, which will permit its selective deletion in SN dopamine neurons. A second central current issue in the neurobiology of GDNF is whether GFRal regulates dopamine neuron development not only in cis (cell autonomously), but also in trans, from postsynaptic striatal neurons. This issue will be addressed with both a selective deletion of GFRal receptors in striatal neurons, and a selective overexpression using a double transgenic approach. These investigations will define the roles of these important signaling molecules in the normal development of SN dopamine neurons, and will provide a basis for their potential use in neuroprotective approaches for Parkinson's disease.